A roof that only leaks in heavy or wind-driven rain usually has a small gap or worn spot that light rain sheds fine, but a downpour overwhelms. The common causes are failing flashing, clogged gutters or valleys that back water up, wind driving rain sideways under tiles or shingles, or ponding on a flat roof that only builds in a hard rain. The leak is real either way. It just needs enough water to reveal itself, which is why it hides in a drizzle.
A roof sheds a light rain easily, so a small weakness never gets tested. Turn that into a heavy downpour and the picture changes. Water runs faster and deeper, backs up behind any clog in a gutter or valley, and finds gaps that a gentle rain flowed right past. Wind makes it worse, pushing rain sideways and up under the edges of tiles and shingles that only ever face straight-down water in a calm rain. On a flat roof, a hard rain can pond water in a low spot deep enough to reach a seam that a light rain never touches.
So the pattern is a clue, not a mystery. It points to a specific weak spot, usually at the flashing, the valleys, or the drainage. Our leak detection traces it to the source, and roof repair or flashing repair closes it.
| Cause | Why only in heavy rain |
|---|---|
| Failing flashing | A small gap only a downpour overwhelms |
| Clogged gutters or valleys | Water backs up under the roof |
| Wind-driven rain | Pushes water sideways under edges |
| Flat roof ponding | Water pools deep enough to reach seams |
Because the leak hides most of the time, it is tempting to ignore it, but a spot that leaks in every heavy rain is soaking the same wood over and over, which is how a small gap becomes rot, as our page on delaying a repair explains. Clearing gutters and valleys is worth doing, since backed-up water is a frequent culprit, and staying ahead with roof maintenance catches these before the season. If it keeps happening, a roof inspection finds the gap the downpour is finding. Related: why a roof leaks with no rain.
Trace it before the next storm. Call (310) 806-5959 or request an estimate below.
Tell us when it happens. We find the gap and give you a written price, with no obligation.